In a coal-fired boiler of the type used in a powerplant to generate steam for the production of electricity, it is standard to provide automatic equipment which periodically or continuously empties the coal ash out of the bottom of the boiler. The hot ash is dumped straight into a water-filled quenching pit, quenching and cooling it instantly sufficiently to handle it. A conveyor transports the wet ash to a storage facility, bricketing operation, or other facility.
The conveyor for such a task must therefore be able to move this heavy and wet bulk material up from below the liquid level in the quenching vessel and carry it some distance. The wet ash is quite abrasive also, so it is very difficult to transport. Hence recourse has been had to scraper-type conveyors.
One such conveyor is described in German patent document No. 2,620,836. In this arrangement the scraper chain is looped at one end over a drive wheel that is outside the quenching vessel, is deflected over several guide wheels submerged in the vessel, and is looped over at least one deflecting roller, so that the submerged portion of the underneath stretch of the chain can sweep the floor of the quenching vessel. The two end rollers, that is the drive and deflecting rollers, are both above the quenching vessel so that the chain travels over a long path having at least five straight stretches, with the lower stretches--one horizontal along the vessel floor and one inclined up out of the vessel--serving to convey the ash by pushing it along in an appropriate channel. This guiding structure is quite complex and must operate as mentioned above in the presence of water and abrasive grit, so that service problems are considerable.
The chain of such a conveyor must naturally be kept taut. To this end a standard spring-loaded tightener is associated with one of the guide or deflecting wheels. The tension must be tight enough that the drive wheel, normally a faceted structure whose sides correspond to the links of the chain, does not slip on the chain unless tension in the chain is so great as to risk breaking it. The problem with such an arrangement is that the loading of the conveyor is not uniform; part of the time it will run virtually empty and part it will be very headily laden, depending on operating conditions of the respective boiler or boilers. The spring of the tensioner is normally adjusted for the maximum load, so that when the device is partly empty the chain is unnecessarily taut and the bearings for the various chain-support wheels are unnecessarily loaded.
In general the known ash-removal conveyors are not responsive to working conditions. They are normally set to operate with a given tension and at a given speed, both calculated to be effective with the greatest load the chain is expected to carry. An electrical system is described in German Pat. No. 1,456,925 for varying conveyor speed in response to some load conditions, but this system responds too slowly to operating conditions and requires complicated load-sensing means.